Native BigInts and Base64 in JavaScript

It's been native BigInts all week long, all week strong and today I'm tying it all in
a nice pretty bow with the final installment:
How to encode BigInts to Base64, and back again.

If you need to handle signed big ints look at the two articles on converting to
and from hex below, that's where all of that is.

With that said, this is a pretty easy - and it's old school.
Despite all the advances in JS most of the old thorns remain
and, somewhat surprisingly, hex and binary strings are still
the easiest way to get some of the most basic stuff done.

Note: For example's sake we're going to use small numbers here,
but don't let that fool you, these tiny functions are built for the big ones!